TORONTO  Melting ice is opening up the Northwest Passage and reviving a dispute between the United States and Canada over who controls the potentially lucrative shipping route.

The United States calls the passage an international strait, open to all. Canada claims control because it considers the passage an internal waterway, like the Mississippi River.

Until recently, the decades-long dispute has been mostly academic; thick sea ice blocks the passage for about 11 months of the year. But as global temperatures rise and polar ice caps melt, the ice-free season may lengthen, making the Northwest Passage a viable shipping route within decades or, the U.S. Navy says, even a few years.

Satellite photos show the ice cover in the Arctic Ocean is shrinking by about 3%-4% each decade, says John Falkingham, chief of ice forecasting for the Canadian Ice Service. The melt has accelerated, he says, to a rate of about 8% per decade since 2000. But because Arctic currents push drifting ice toward the Canadian archipelago, he predicts more ice in the passage for the near term. However, Falkingham says, "at the end of the century, there could be an extended summertime shipping season."

Others expect faster change. A 2001 U.S. Navy report predicted that within 10 years, the passage would be open to non-ice-strengthened vessels for one month a year. Only icebreakers and specially made ice-hardened ships now travel the passage, mainly for military purposes and scientific research.

7,900 miles vs. 12,600 miles

A reliably ice-free Northwest Passage could be a far shorter alternative to the Panama Canal. A 12,600-nautical-mile trip from Europe to Asia via the Panama Canal would be 7,900 nautical miles using the Northwest Passage. That would save hundreds of thousands of dollars for shipping companies.

"People are looking for new ways to get across the North American continent," says Garrett Brass, executive director of the U.S. Arctic Research Commission in Arlington, Va. "It would be a very attractive way to move Alaskan oil to the East Coast."

Generations of explorers dreamed of the Northwest Passage, a shortcut between the Atlantic and Pacific oceans. Many perished searching for it. In 1906, Norwegian Roald Amundsen became the first person known to complete the passage by sea in his converted herring boat. In 1969, the U.S. tanker Manhattan became the first commercial vessel to make it through the passage, accompanied by a Canadian icebreaker.

Since then, only a few ships have followed. The United States says those multinational voyages clearly mark the Northwest Passage as an international strait.

Canada claimed the passage as an internal waterway in 1973.

The United States generally supports maximum freedom of the seas. U.S. officials worry about what sort of precedent the Northwest Passage could set for international straits in global hot spots, such as the Strait of Hormuz near Iran and the Strait of Malacca between Malaysia and Indonesia. "We don't want people closing the Straits of Gibraltar," Brass says.

For Canada, the Northwest Passage is a symbol of national sovereignty, which Canadians guard fiercely. The Canadian national anthem boasts of the "True North, strong and free."

"Some issues go beyond rationality," says Rob Huebert, associate director of the University of Calgary's Center for Military and Strategic Studies. "Any sign of an affront to northern sovereignty is absolutely guaranteed to get on the front page of all the newspapers."

That's what happened in January, when David Wilkins, the U.S. ambassador to Canada, casually mentioned the two nations' "agree to disagree" policy on the Northwest Passage. "We don't recognize Canada's claims to the waters," Wilkins said at a University of Western Ontario forum on Jan. 25.

Prime Minister Stephen Harper responded the next day. "We have significant plans for national defense and for defense of our sovereignty, including Arctic sovereignty," Harper said at his first news conference after his victory in Canada's Jan. 23 election. "It is the Canadian people we get our mandate from, not the ambassador from the United States."

'It's just a matter of time'

Michael Byers, who holds the Canada Research Chair in Global Politics and International Law at the University of British Columbia, says the agree-to-disagree policy "has become unsustainable because the ice conditions are changing so dramatically."

That's one reason Harper is determined to assert control over the passage. Harper's election campaign included promises to invest $5.3 billion Canadian dollars in northern defense, including three armed ice breakers, a deep-water port near Iqaluit by the eastern entrance to the passage and more soldiers stationed in Canada's far north.

The Northwest Passage isn't the only contentious Arctic issue. Canada and Denmark waged a war of words last summer over Hans Island, near the Arctic Circle. Both nations claim the barren island, which is about the size of a football field. Why do they care? Because Hans Island sits in the midst of what could become a busy shipping route once more polar ice melts.

Whether this Arctic showdown ever heats up depends on how soon the ice melts in the Northwest Passage. Brass says another Arctic waterway, the Northeast Passage along the northern coast of Russia, could emerge as a viable shipping route first.

Despite Falkingham's prediction that there could be an initial ice buildup, he and others agree that an ice-free Northwest Passage is not a matter of if, but when.

"A commercially viable passage is coming. It's just a matter of time," Byers says. He says the historically close U.S.-Canada trade relationship will motivate both to chart a course toward compromise on the waterway: "I think cooler heads will prevail," he says.